1. Field of the Invention
The present invention relates to an electrophotographic image processing method and an electrophotographic apparatus for carrying out the method, wherein halftone processings are performed through use of halftone spots each formed of a plurality of dots, as well as to a recording medium having recorded thereon a program to be used for carrying out the image processing method. In this specification, the word “dot” implies defines “pixel” as a unit cell in electrophotographic apparatuses.
The present application is based on Japanese Patent Applications No. Hei. 11-48563 and 11-330292, which are incorporated herein by reference.
2. Description of the Related Art
In an electrophotographic apparatus, such as a color printer or a color copier, a color image is reproduced by utilization of cyan toner, magenta toner, yellow toner, and black toner. Particularly, among color printers, some page printers—which form a latent image on a photosensitive drum by utilization of a laser beam, develop the latent image by use of charged toner, and transfers an image formed from the thus developed toner onto transfer paper—an change an area to be exposed by the laser beam in various manners and within a region of dots (hereinafter referred to as a “dot region”). Thus even when the number of dots per unit area is small, those page printers can reproduce a color image with high resolution and high gradation.
In such a color electrophotographic apparatus, a dithering method has been widely utilized as the binary-coding technique for reproducing the halftone of a gray-scale image. According to the dithering method, by reference to conversion tables which are called dither matrices or threshold-value matrices and which define the correspondence between halftone data and image reproduction data, a determination is made as to whether color spot is displayed in each of dot or not. A dot is “ON” when color spot is displayed and “OFF” when color spot is not displayed. Halftone spots are produced by one dot or some adjacent dots turning “ON”, and halftones of the images are reproduced on the basis of the sizes of halftone spots.
Dots are arranged in the direction of primary scanning in which a laser beam is moved for scanning (hereinafter referred to simply as a “primary-scanning direction”) and in the direction of secondary scanning in which transfer paper is fed (hereinafter referred to simply as a “secondary-scanning direction”). As some dots become “ON” and thus form the “core of the growth” of halftone spots. As the gray-scale level of the halftone data is increased further, the number of “ON” dots is eventually increased, thus gradually enlarging the size of halftone spots.
Through use of a multivalued dithering method which is an extension of the dithering method, toner can be adhered in only a predetermined area of the dot region. The multivalued dithering method enables reproduction of an image having a higher gray-scale level. As mentioned above, even in the case of an electrophotographic apparatus as typified by a laser printer and uses a small number of dots per unit area (for example, 600 dpi), the multivalued dithering method enables reproduction of an image with high resolution and high gradation.
Such an electrophotographic apparatus encounters a problem of stripes or inconsistencies (a stripe pattern oriented in a certain direction) arising in a reproduced image. The stripes or inconsistencies are attributable to various reasons, and diagnosis and solution of all the reasons for the stripes or inconsistencies is difficult.
FIG. 1 is an illustration showing the combination of the angle of a cyan screen, the angle of a magenta screen, the angle of a yellow screen, and the angle of black screen, which has conventionally and widely been used in an electrophotographic apparatus of the background art. As shown in the drawing, according to the conventional technique, the angles of four color screens are set such that the angle of the yellow (Y) screen is set to 0°; the angle of the cyan (C) screen [or the angle of the magenta (M) screen] is set to 15°; the angle of the black (K) screen is set to 45°; and the angle of the magenta (M) screen [or the angle of the cyan (C) screen] is set to 75°. It is also known that, if the screen angles of the halftone spots are shifted in order to prevent chromatic misregistration, a so-called moiré pattern appears. It has empirically been acknowledged that a shift of angle of about 30° between two color screens is optimal for increasing the spatial frequency of the moiré pattern, to thereby render the moiré pattern inconspicuous. Among the four colors, yellow is the least noticeable to the human eye. Therefore, the angles of color screens other than the yellow screen; that is, the angle of the cyan screen, the angle of the magenta screen, and the angle of the black screen, are shifted from one another by 30°. Further, the angle of the black screen, which is most noticeable to the human eye, is set to 45°, so as be most distant from a longitudinal angle of 0° and a horizontal angle of 90°, which are easily recognized by the human eye. The angle of the cyan screen is set to 15°, and the angle of the magenta screen is set to 75°, or vice versa. As mentioned above, since yellow is least noticeable to the human eye, the angle of the yellow screen is set to 0°.
The foregoing method is employed in order to prevent occurrence of stripes or inconsistencies, which would otherwise be caused by shifting screen angles of halftone spots from each other. Under this method, stripes or inconsistencies, which would be caused by a moiré pattern, are prevented by means of shifting angles of color screens by about 30°.
There may be a case where stripes or inconsistencies are caused by a reason other than that mentioned previously. A predetermined area within dots is developed through use of the multivalued dithering method, to thereby set the centroid of the halftone spot to an arbitrary position. This technique is described in detail in Japanese Patent Application No. Hei. 11-28666 which is incorporated herein by reference. Under this method, there may be a case where only the portion of the dots of the halftone spot where narrow dots are present is developed.
Such a narrow area is formed as a result of a photosensitive drum having been exposed to a laser beam for a short period of time, and hence the resultant latent image of the narrow area has an insufficient surface potential. Accordingly, the toner adhering to such a latent image is developed to a poor extent, and the area which is actually developed through use of toner tends to become much narrower. As a result, even in a developed image having a single gray-scale level, variations or inconsistencies arise in the area of halftone spots constituting the image. For example, stripes or variations, which would be caused by variations in the area of halftone spots, arise in an image in which a bright color is to be reproduced over a certain area.